Municipal and industrial sludge and waste and other sources of waste-products of primarily organic origin such as by-products from gardening, agriculture, forestry, timber industry and the like, have over the years been the subject of increasing interest as possible starting materials for the production of e.g. CO2-neutral fuels such as biogas or bioethanol.
Many known methods for production of CO2-neutral fuels based on such organic waste or biomass include a pre-treatment step employing some kind of Thermal Hydrolysis Process (THP) and an anaerobic digestion.
When organic waste or biomass is subjected to THP processes, this will in most cases result in a release of at least part of the otherwise organically bound phosphorus compounds. Thus, the material resulting from such processes will very often comprise relatively high concentrations of in particular orthophosphates. This is in particular true for THP-treated municipal and industrial sludge.
Thus, in addition to the desired CO2-neutral fuels, these processes will after the anaerobic digestion result in solid, semi-solid and wastewater effluent fractions containing a considerable amount of phosphate originating from the source of organic material if no measures are taken to reduce the amount of phosphate in the course of the process. Various methods have previously been developed to recover some of the phosphate from the wastewater effluent in the form of pure magnesium ammonium phosphate (MAP) which is useful as fertilizer in the field of agriculture.
Known processes for recovery and precipitation of phosphate in the form of magnesium ammonium phosphate (MAP, NH4MgPO4, Struvite) from such processes include an addition of magnesium chloride to the solid or semi-solid fraction leaving the digestion tank wherein the anaerobic digestion is performed. These processes typically also encompass one or more separation steps where the solid or semi-solid waste are separated from the wastewater effluent prior to precipitating MAP from the then formed wastewater effluent fraction.
These processes, which are similar to methods for removal of phosphate applied also to different kinds of wastewater, have been known for some years, and are normally applied to the resulting waste product at the very end of the relevant procedure, e.g. to the product resulting from the treatment of the applicable biowaste in a digestion tank.
One such common method for recovering the phosphate in wastewater is by bringing the phosphate to react with ammonium already present in the wastewater and adding magnesium to form the precipitate NH4MgPO4.
EP1241140 describes a process called ‘AirPrex’ for the controlled formation and removal of struvite directly from digested sludge. In the AirPrex process the digested sludge is led through a reactor tank where air is supplied and magnesium is added as magnesium chloride (MgCl2). Air is supplied in order to raise the pH value (by CO2 stripping) and to obtain sufficient mixing of the sludge and the added magnesium chloride. The formed struvite is intermittently tapped from the (conical) reactor bottom. In a second tank, smaller crystals of struvite are allowed to settle.
The pH necessary for the precipitation of struvite (MAP), normally in the range of 7.6 to 8, is typically reached by the addition of alkaline agents, e.g. a sodium hydroxide solution or other alternative similar measures. Apart from the AirPrex process, several other alternative processes for recovery of struvite are known in the art.
US2012/0261334 discloses inhibition of formation of scale in a wastewater treatment system upstream of a struvite precipitation reactor by injection of CO2. The injection may be controlled based on one or more of the variables pH, fluid flow and fluid pressure. The injected CO2 may subsequently be stripped, at the precipitation reactor to enhance struvite production.
Another known alternative is the Crystalactor® technology, which was originally developed for water softening purposes. It was later recognized that the reactor could be used for the crystallization of a variety of (heavy metal) carbonates, phosphates, halides and sulfides in the process industry. Phosphate may be recovered in the form of struvite. In essence, the Crystalactor® is a cylindrical vessel which is partly filled with a suitable seed material. Feed, reagent and recirculating solution are pumped upward through the particle bed at a rate to maintain favorable mixing and supersaturation conditions. Effluent overflows the top of the reactor whereas the seed material in the bed grow into pellets through crystallization. As the pellets become progressively heavier they gradually move towards the bottom of the bed. Periodically, the lower portion of the bed is discharged into a pellet container and fresh seed material is added without interrupting the operation.
U.S. Pat. No. 8,445,259 discloses an apparatus and a method for treating organic sludge, wherein the sludge is first dewatered; the dewatered sludge is passed through a thermal hydrolysis reactor to hydrolyze polymers contained in the dewatered sludge; the hydrolyzed sludge is passed through a digester to digest the hydrolyzed sludge anaerobically; the digested sludge is again dewatered to form a dewatered cake and a solution; and then the solution is passed through a crystallization reactor to crystallize and remove phosphorus and nitrogen in the solution. In the crystallization step is typically added magnesium and an alkaline solution.
EP 1 364 915 A1 discloses a method for reducing phosphate from the liquid phase of digested sewage sludge, wherein wastewater is fed to aerobic treatment after the anaerobic treatment and sludge recycled from a settling tank is subjected to anaerobic treatment. The water fraction from a second solid/liquid separation step is then fed to an apparatus for removing phosphate, for example a MAP reactor.
WO 2009/112208 discloses a method for wastewater treatment and a wastewater treatment plant for this purpose, in which hydrolyzed and subsequently anaerobically treated sludge is fed to a separate precipitation unit in order to remove phosphate. In this method magnesium ammonium phosphate (MAP) is precipitated from the hydrolyzed and anaerobically treated sludge by addition of magnesium salts with the setting of a pH from 7.5 to 7.8.
In contrast to the above methods, WO 2013/034765 describes a method for separation of phosphate from a process flow, in which the separation is carried out after a thermal hydrolysis step but before the anaerobic digestion step, in the form of removal of Struvite (MAP) from the process flow by precipitation. A magnesium-containing precipitant is added to the process flow and a sub-quantity of the upstream anaerobically digested sludge in the form of a separated liquid phase is recirculated to the process flow after hydrolysis but before or during the step of removal of phosphate, to provide ammonium for formation of MAP. It is described that the removal of phosphate in the form of MAP crystals prior to the digestion step as compared to after the digestion step has the advantage that the ratio of crystal structure to sludge particle structure makes it possible to improve MAP crystal removal, for example by means of a centrifuge decanter, from the process flow, thus resulting in a higher MAP yield, with a relatively fine crystal structure. The process is described as being especially advantageous when the hydrolysis is performed at a temperature in the range of 70° to 90° C., and results in a hydrolyzed product with a pH in the range of 10 to 12.
Phosphorous is characterized as a limited resource on earth, relatively scarce and not evenly distributed around the planet. Today the phosphorous for fertilization comes from mining of phosphate rock as the guano reserves are depleted. Some researchers estimate that also the phosphate rock reserve will be depleted in 50-100 years. A large part of the until now mined phosphorous have ended up in the water environment or have been deposited with waste, thereby becoming unavailable for reuse. Recovery of phosphate becomes increasingly urgent.
Considering the increased need for effectively recovering phosphate from biomass products in general, including e.g. wastewater sludge, and municipal or industrial waste, there is a continued and increasingly urgent need for the development of efficient and energy saving methods for treating biomass and concomitantly recovering phosphate. For instance, there is a need for reducing the use of water, energy and costly chemical agents.